A locus of a contact point of external gear tooth profiles making continuous contact generally forms a lemniscate loop curve and if this loop is flattened toward a center line between axes and coincides with the center line, the contact point moves on the center of both gears. Therefore, the tooth profile has a contour that achieves rolling motion, which corresponds to a pitch curve of non-uniform speed gear (non-circular gear).
A contour of a rolling contact wheel which has a constant center distance and an angular velocity ratio that varies during rotation is a non-circular curve and a non-circular gear is a gear provided with teeth using this non-circular curve as a pitch curve to ensure transmission. A transmissible tooth profile is generally provided on the pitch curve. This tooth profile may be involute, cycloidal/trochoidal, Novikov, or a combination thereof, each of which has drawback and advantage.
For example, a helical gear positive displacement flowmeter described in Patent Document 1 uses a single-point continuous contact tooth profile of a single curve. The single-point continuous contact tooth profile is ideal for a rotor tooth profile of a flowmeter since higher surface pressure bearing strength is expected due to convex/concave contact of tooth surfaces and no trapping phenomenon occurs which is disadvantageous to a flowmeter. However, the single-point continuous contact tooth profile represented by a sine-curve gear has a helical shape due to a contact ratio and the tooth height ratio is limited to 0.785 m, i.e., nm/4 (m: module) if formed by a single curve. This is disadvantageous to a rotor of a flowmeter in aspect of function or production.
On the other hand, a pitch curve of a non-circular gear is considered as an ideal tooth profile curve that enables the continuous contact and the rolling contact instead of slide contact between tooth profiles as a gear tooth profile. It is assumed that a tooth profile of a flowmeter rotor is formed by a pitch curve (rolling-contact curve) of an oval gear that is a type of non-circular gears. In this case, it is known that the tooth profile does not act as a gear since a tooth profile for certainly transmitting the rotation is not included.
Patent Document 1: Japanese Patent Publication No. 3310239
However, in the case of rotors of a positive displacement flowmeter housed as a pair in a measuring chamber of the flowmeter to transmit rotation through energy of fluid, although the torque transmission is switched at a position of the equal diameters where the torques are theoretically equalized and a tooth surface on the opposite side comes into contact, if an appropriate gap considered as an apparent backlash may be formed, abnormal engagements may be reduced and the tooth profiles making the rolling contact may be realized. However, in a conventional positive displacement flowmeter, the technical idea as described above for using the tooth profile with the rolling-contact curve as the rotor does not exist and has not been realized.
The present invention was conceived in view of the above situations and it is therefore the object of the present invention to provide a positive displacement flowmeter having a pair of non-circular gears making rolling contact without slip on a pitch line.